Consumer Reports: Do car MPG ratings promise too much?

July 27, 2013

When comparing the fuel economy of cars, consumers often rely on window stickers that display miles-per-gallon estimates from the Environmental Protection Agency. But Consumer Reports' testers have found that the figures for certain vehicles can be far higher than many drivers will actually get. And the largest differences involve some of the most fuel-efficient cars, particularly hybrids.

When Consumer Reports compared the EPA estimates of 315 vehicles with the results of its real-world fuel economy tests, testers also found notable gaps in cars that use small turbocharged four-cylinder engines, intended to provide the power of larger engines and the gas mileage of smaller ones.

Sticker Shock

In Consumer Reports' testing, hybrids generally get some of the best overall gas mileage numbers in their classes, led by models such as the Toyota Prius (44 mpg) and hybrid versions of the Honda Civic (40), Ford Fusion (39) and Toyota Camry (38). But an owner expecting to get the same mpg shown on the window sticker and in advertising might be disappointed.

Of the recently tested hybrids, 55 percent fell short of their EPA combined city/highway estimates by 10 percent or more, with hybrids built by Ford showing the largest discrepancies.

At 34 mpg overall, the Lincoln MKZ Hybrid is invitingly thrifty. But it gets 11 mpg, or 24 percent, less than its 45-mpg EPA figure. The C-Max and Fusion hybrids fall 10 and 8 mpg, respectively, below their advertised 47 mpg. Similarly, the Volkswagen Jetta Hybrid also falls 8 mpg short. For consumers who buy the MKZ, for example, that can amount to paying $1,510 more in gas over five years than they might have expected (assuming gas costs $3.50 per gallon and the car is driven 12,000 miles annually).

Similarly, Consumer Reports found that 28 percent of cars with small turbo engines also fell short of their EPA estimates by 10 percent or more. Those include models such as the Buick Encore, Ford Fusion and Nissan Juke.

What's Going On?

EPA estimates don't always reflect real-world driving performance largely because they are based on outdated tests designed to measure vehicles with conventional powertrains in particular driving situations rather than today's increasingly sophisticated gas/electric systems. In fact, according to Mike Duoba, a research engineer at Argonne National Laboratory who works on keeping the tests up to date, the EPA tests "were originally designed to test emissions, not fuel economy. They wanted to test a variety of speeds and accelerations."

The EPA test for city fuel economy is conducted at very low speeds, with gentle acceleration and minimal idling. The highway test includes quite a bit of stop-and-go driving, with a maximum speed of 60 mph and an average speed of 48.

Hybrids are most efficient in those conditions. With a light foot on the throttle, the latest models can often cruise in electric mode up to about 60 mph, so they can perform portions of the EPA tests without consuming a drop of gas. By contrast, Consumer Reports' highway mpg tests are performed by driving at a steady 65 mph, reflecting a driver cruising on an interstate highway. In that situation, a hybrid is constantly running its gas engine, so it doesn't get the full benefit of using its electric power. Thus, it gets fewer mpg than in the EPA test.

Similarly, small turbocharged engines rarely need to spool up their turbos to develop sufficient power for the EPA tests. Consumer Reports' test calls for larger throttle openings so that cars can accelerate from, say, 20 to 40 mph within 500 feet, which results in more turbo use and more air and fuel being pumped into the engine.

Consumer Reports has discussed its findings with the EPA, and the agency says it is reviewing its tests and is considering updating them. Meanwhile, consumers should be aware that they might not get the efficiency promised on the window sticker.